In certain applications it is necessary to deliver a high torque as the rotational speed of a drive roll goes from zero to a maximum RPM and back to zero doing many cycles per minute. By way of example, in a system for the manufacture of plastic film bags such as garbage bags, trash bags, and the like, it is necessary to precisely position the plastic bags delivered from a bag machine to an exact position on a vacuum/air conveyor for transfer to a bag folder. Examples of such a bag making system are disclosed in my related applications Plastic Film Air Table Conveyor, Ser. No. 099,275 and Flexible Material Folding Apparatus Ser. No. 099,277 concurrently filed herewith and incorporated herein by reference thereto. In a bag making system of the type disclosed therein, the torque applied to the driven roll on the vacuum/air table is in the order of plus and minus 500 in. lbs. as the rotational speed goes from zero to 1200 RPM and back to zero 120 times per minute. Five hundred in. lbs. of torque must be generated to index the conveyor belts within the 300 milliseconds cycle time allocated for a 120 bag per minute production rate.
Various types of indexing drives have been used in the past for bag machines. One example is the crank, rack-pinion, clutch-brake system commonly used for bag machine indexing drives. The disadvantage in such drives is that the clutch brake is subject to wear and loses accuracy and it also has a high inertia. Also in such drives the index is limited to 50% of the cycle and thus are not suitable for the present application where more time is desired for indexing. Another example of an indexing roll drive system is disclosed in U.S. Pat. No. 4,192,705 which discloses a short dwell mechanical indexing drive system for intermittently moving a web of thermosealing material through a bag machine with the web advancing or indexing movement of each cycle being in excess of 180.degree. of the cycle. One of the problems with mechanical index drives is that the speed at the torques required adds too much inertia to the system. Electric servo index systems have been used in bag machine lines; however, they require an increase in power to cover both motor inertia and vacuum table inertia.
The hydraulic index drive system of the present invention has numerous advantages. A small control force yields a large output torque. The system has both index length flexibility and cycle time flexibility. The drive can be coupled directly to the vacuum table drive shaft through a flexible coupling eliminating inertia for pulleys and belts or gears. The drive is inexpensive as a commercial pump and motor can be used. The rotor of the pump adds little inertia to the system. No clutch-brake is required. There is an overload release at maximum pressure and the system requires a relatively small motor drive.
The present invention incorporates an adaptor housing between the variable volume and the hydraulic motor having a fixed volume which is of unique design. The adaptor includes a special flushing arrangement to divert flow so as to purge air within the pump during setup. The adaptor is specially designed to pass oil between the motor and pump with minimum resistance and includes a relief valve. Of the approximately five horse power required to drive the hydraulic index drive system one horse power is converted into heat. The cooling in the present system is provided by a water cooled system for the adaptor. Since oil is compressible, the working volume of the system is kept at a minimum. Only approximately three cubic inches of oil are subjected to high pressure and the pump displacement needed is only about 1.29 cubic inches.